Refrigeration systems of the general type contemplated by the present invention comprise a closed circuit containing a salt solution as the refrigerant composition. Cooling is effected by vaporizing liquid refrigerant within an evaporator coil or the like which forms part of the closed circuit. Air or other fluid medium is passed over the coil and thereby becomes chilled as it provides heat of vaporization to the refrigerant. The resulting refrigerant vapor passes to an absorber section where it is contacted with and absorbed into a stream of liquid absorbent. Heat liberated by the absorption process is removed by heat exchange with an external cooling medium. The resulting solution which has high refrigerant content passes to a generator section where it is heated by an external heat source to expel refrigerant vapor. The resulting solution which has low refrigerant content returns to the absorber section. The refrigerant vapor is condensed by heat exchange with an external cooling medium and then again passes to the evaporator section.
The operation of a typical absorption circuit depends, among other things, on the reduction of the vapor pressure of the refrigerant as it is absorbed into the absorbent. The low pressure which is produced in the absorber section is transmitted to the evaporator section and causes the liquid refrigerant to evaporate until its temperature drops to a value at which the refrigerant vapor pressure approximately equals the pressure in the absorber section. That is, the refrigerant temperature varies directly with absorber pressure. It is evident, therefore, that the cooling capacity of the circuit is directly related to absorber pressure, and for this reason it is common practice to employ an absorbent which has a low vapor pressure at the operating temperature of the absorber section.
It is recognized in the art that one way of obtaining a low absorber pressure, and thereby achieving greatercooling capacity, is to employ a more highly concentrated salt solution. On the other hand, it is also recognized that this advantage is offset by the fact that the highly concentrated salt solution has a greater tendency to crystallize, so of course the maximum concentration is limited by the solubility of the salt.
These disadvantages are aggravated in an air-cooled refrigeration system, such as an air-cooled air conditioning unit employed to cool the interior of an enclosure, for two reasons. First, a higher absorber temperature is required for a given evaporator temperature, and this in turn calls for the use of a salt solution having a particularly low vapor pressure, so low that the solubility characteristics of the commonly employed salts preclude obtaining the desired vapor pressure. Second, even if the first problem is overcome by employing a particularly soluble salt, the crystallization temperature of the concentratedsolution may be so high that the solution will crystallize when the system is shut down. For example, a solution having a crystallization temperature of 80.degree. F. will not crystallize at the operating temperature, say 130.degree. F. of the system, but after shut down the solution will cool toward ambient temperature which may well be below 80.degree. F.
Further problems arise in selecting a refrigerant composition when the temperature of the heat source for operating the vapor generator is relatively low, as would be the case if the heat source is a stream of solar heated fluid at a temperature of about 200.degree. F. That is, the refrigerant composition must be one which, under the pressure existing in the generator, is capable of boiling at the low temperature of the heat source.
Many absorbent refrigerant compositions have been disclosed in the prior art. U.S. Pat. No. 3,524,815 (Hensel et al) discloses an aqueous lithium bromide/lithium iodide solution which has a sufficiently low crystallization temperature that is suitable for use in an ambient air-cooled system. U.S. Pat. No. 2,802,344 discloses a variety of electrolyte solutions as refrigerant compositions, including alcohol solutions of various inorganic salts.